THE FIRST AND SECOND GENERATIONS

Two 'Generations' of angels exist in TCP.  The 'First Generation' is classified as such because its members were created directly by God. 

Members of the Second Generation were born to intermarriage between angels of the first (the Third Generation will be created by intermarriage of the Second and so on).  In TCP, angels having relationships is not considered taboo; it is in fact expected and planned
that certain angels will marry and have children.  However, a female angel is only able to have one child, so all angels are only children.  The one exception to the only child rule is the case of Anael and Gabriel, members of the First Generation.  They are actually twins, because the embryo from which they were born split as it developed. 

The members of the first Generation are generally drawn to each other by the nature of their spheres.  Opposing and complimentary spheres are usually matched equally--for example, an angel presiding over the Heart would just as easily be drawn to an angel
presiding over the Mind as to an angel of Emotion.  

Relationships that produce children are severely restricted by the Law of Predetermination.   This is because not only can a female angel have only one child, she can only be pregnant by a certain person.  This ensures a high level of population control, and it is a defining aspect of angelic society.  As one might expect, this law is unpopular with some people, but ultimately it's effective: for whatever reason, most 'perfect' matches do end up happening.  That doesn't necessarily ensure the longevity of the relationship or the happiness of the child, though...